1. Field of the Invention
The present invention relates to systems and methods that form images from coherent laser radiation, and in particular relates to systems and methods for truncating such images without a substantial modification in intensity of the truncated image due to diffraction. The invention has industrial utility in a number of fields, including laser thermal annealing (LTA) (also referred to as “laser thermal processing” or “LTP”) used in semiconductor device manufacturing.
2. Description of the Prior Art
LTA involves irradiating a semiconductor substrate (“wafer”) with a scanned beam of radiation that rapidly brings the substrate surface temperature from a relatively low temperature (e.g., 400° C.) to a relatively high temperature (e.g., 1,300° C.) in order to activate dopants in the substrate. Only the material very close to the top surface of the substrate is heated to the relatively high temperature during the dwell time of the scan. Thus the top surface is cooled by the low temperature in the bulk of the substrate almost as quickly as it was heated. Typically, LTA systems produce such a thermal cycle in a time span of a few milliseconds or less. To effectively anneal the entire surface of a substrate, the peak irradiation seen at each point on the substrate must be relatively uniform, e.g., within +/−1%. Further, the irradiation must be performed in a manner that keeps thermal stresses in the substrate below levels that could lead to breakage or slip within the crystal structure.
The “Detailed Description of the Invention” section includes a “Definitions” section that defines many of the terms used hereinbelow. FIG. 1 is a schematic plan view of a wafer 10 undergoing LTA, and FIG. 2 is a cross-sectional view of FIG. 1 taken along the line 2-2. Wafer 10 has an upper surface 12 and an outer edge 13. Upper surface 12 is typically divided into two zones: a thin, annular (e.g., 3 mm) edge exclusion zone 14 surrounding a central product zone 15 wherein the semiconductor devices are formed. In exclusion zone 14, no product, or product that zone would yield, is expected.
In LTA, a narrow image 20 (e.g., a line image) is formed by a LTP laser beam 26. LTP laser beam 26 is formed, for example, by a radiation source 28 (e.g., a CO2 laser) generating a coherent radiation beam 29 and passing the radiation beam through an optical system 32 that includes an optical relay (not shown). Image 20 scans back and forth across wafer surface 12 in the direction illustrated by the two arrows 3 and 4, e.g., by moving the wafer relative to the line image. LTP laser beam 26 is preferably incident on wafer surface 12 at an incidence angle θI relative to surface normal N (FIG. 2), and more preferably is incident at or near Brewster's angle, which is about 75° for a silicon wafer and infra-red radiation at a wavelength of 10.6 microns. Due to of the incident angle θI of laser beam 26, in the area close to position A the laser beam strikes the thin vertical edge 13 of the wafer as the beam transitions across the edge. In this case, energy is absorbed both through wafer top-surface 12 and through wafer edge 13, which drastically increases the temperature and the thermally induced stress in this area. Once the thermal stress exceeds the wafer's elastic limits, breakage occurs.
With reference to FIG. 1, when the center position O on the wafer is irradiated by laser beam 26, the intense heat generated at this center position dissipates in five directions, forward and backwards in the scan path as well as to both sides of the scan path, as well as down into the body of the wafer. This minimizes thermal gradients and stress levels so that breakage tends not to occur when the beam is in the central area. However, at or near the wafer edge locations near position A, if the scanning velocity of image 20 is low enough, and if the annealing temperature is high enough, wafer breakage can occur. In this case, the region near the edge is heated both through the wafer upper surface 12 and through the wafer edge 13, which exacerbates the temperatures and stress in the vicinity of the edge. A second type of wafer edge breakage occurs at the ends of the scan path near points B and C. Breakage occurs here because as the heat dissipation path forward of the scanned beam grows shorter as the edge is approached the heat is concentrated into an increasingly smaller area near the edge which results in the build-up of a significant amount of heat at the wafer's edge. This type of breakage can also occur at or near the wafer edge between A and B and A and C.
An even worse mechanism for breakage occurs to heat being applied through the edge of the wafer. Given the shallow angle of the beam on the wafer (typically 15 degrees from horizontal) causes the beam intensity to be nearly four times higher on the wafer edge than it is on the top surface of the wafer. Since the beam can not strike the edge of the wafer facing away from the direction of incidence, this breakage mode only occurs on half the wafer, i.e., the half on which the beam strikes the edge.
FIG. 3 illustrates one apparently straightforward way to attempt to solve the wafer breakage issues described above. FIG. 3 is a cross-sectional view of wafer 12 similar to that of FIG. 2, with the addition of an elevated skirt 50 that surrounds the substrate edge. Elevated skirt 50 includes an upper surface 52, a lower surface 53, and a central aperture 54 defined by an inner edge 55. Skirt 50 is supported in a plane above wafer surface 12, with inner edge 55 arranged near wafer outer edge 13. Elevated skirt 50 is arranged so that the obliquely incident LTP laser beam 26 is prevented from irradiating wafer edge 13 by first encountering skirt upper surface 52. This approach at first glance would appear to solve the wafer edge breakage problem.
Unfortunately, the elevated skirt approach introduces two important adverse effects that make it an untenable solution. First, when skirt inside edge 55 is introduced in the path of coherent laser beam 26, diffraction occurs, as indicated schematically by diffracted wavefronts 57. For the laser wavelengths typically associated with LTA (e.g., 10.6 microns from a CO2 laser), the diffraction effect is quite apparent for distances between the skirt and the wafer of 0.2 mm or more. This diffraction of coherent radiation introduces non-uniformities into the unobstructed portion of the laser beam. These non-uniformities create a non-uniform image 20, which results in non-uniform annealing of the wafer near the wafer edge 13.
In product zone 15, adjacent scans 3 and 4 are butted together so that the maximum variation in the peak intensity seen by any point in the product zone is between the maximum intensity and an intensity threshold value ITH. The threshold value ITH depends on the nature of the process and can be, for example in a critical application, 99% of the maximum intensity or, in a less critical application, about 96% of the maximum intensity.
Computer modeling of the diffraction effect indicates that skirt inside edge 55 needs to be closer than 0.1 mm to wafer surface 12 in order to keep the illumination non-uniformity within acceptable limits. However, such close placement is unfeasible in practice. In addition, on side 60 of wafer 10, radiation 62 is reflected from wafer surface 12 and bounces between the wafer surface and the lower surface 53 of the skirt. In addition, the beam reflected from the edge of the skirt is directed back onto the substrate where it interferes with the directly incident beam. These effects pose additional wafer heating problems, which adversely affect the LTP process.
Accordingly, a more effective way of truncating a scanned coherent radiation beam near the edges of the substrate is needed so that the resultant truncated image does not adversely affect the LTP process uniformity or break the substrate.